Thanks for the call, Roger. Sorry I wasn't able to answer.
Here is a picture of my sump setup, taken January 2012, and has remained the same since this picture was taken. The reservoir is the chamber on the far right. The return hose is clamped to the first chamber, where the return pipe is, and is in a position of being higher than the glass--there is no way a siphon could occur.
The reservoir holds approximately 7 gallons of water, but was only filled half way when the last incident occurred. Since the osmolator was having an issue with overfilling, I would only fill the reservoir with no more than 2 gallons of water. After going through the steps in the sticky there wasn't an issue for a couple of weeks, and I thought I was in the clear and filled the reservoir with more water--about 4 gallons--and it was pumped into the sump during the night. I had adjusted the optic eye and the overfill float switch in a manner that didn't allow for much water to be pumped into the sump before activating the kill switch, but this did not work.
I, too, am perplexed. It has been working great the past year, but only began this issue about a month ago.
Thanks,
Derek